1. Field of the Invention
This invention relates to improvements to the crystallization and molding behavior of poly(alkylene terephthalate).
2. Description of the Prior Art
The use of inert particulate nucleating agents to promote crystallization of poly(alkylene terephthalate) is very well known. High mold temperatures of at least 120.degree. C. and optimumly 140.degree. C. in the case of poly(ethylene terephthalate) (PET) have generally been necessary for injection molded engineering applications. In the case of PET, because of its low rate of crystallization at desirable molding temperatures of less than 100.degree. C., it has not attained widespread use. When PET is molded with conventional nucleating agents at temperatures below 100.degree. C., they have poor physical properties, dimensional stability and surface appearance due to insufficient crystallization.
Various attempts have been made to improve the crystallization rate of PET. For example, the use of poly(ethylene oxide) or poly(propylene oxide) having hydroxyl functionality has been suggested in combination with nucleating agents; however, this method is undesirable because the PET reacts with the hydroxyl functionality to give a resin with lower molecular weight and poorer physical properties. The use of low molecular weight inert organic solvents such as benzophenone and phthalate plasticizers such as dioctyl phthalate have been suggested, but is not desirable because they cause processing difficulties due to their high volatility under drying or processing conditions for PET, and tend to migrate to the surface of a molded specimen. Solvents such as dibutyl tin dilaurate have been suggested, but these are toxic materials and would be undesirable.